Metaprogramming: method_missing and __send__

Question

I've found an oddness in Ruby. Using method_missing/respond_to_missing? to dynamically handle method calls initiated via __send__ fails when Kernel defines the same method:

class Testerize

  def method_missing(method, *args, &block)
    if method == :system
      puts 'yay'
    else
      super
    end
  end

  def respond_to_missing?(method, internal)
    return true if method == :system
  end

end

t = Testerize.new
puts t.respond_to?(:system)
# Prints true
t.system
# Prints 'yay'
t.__send__(:system)
# Exception: wrong number of arguments (ArgumentError)

Kernel.system is somehow getting in the mix. Does anyone know what's going on here? I would have expected the :system "message" to get posted to the Testerize instance, hit method_missing, and voila. Why isn't my method_missing getting called when using __send__ when it is with direct invocation?

I'm using Ruby 1.9.3, if that is relevant.

Answer 1

with '__send__' or 'send' we can even call private methods of the object.

In your script do:

t.private_methods.grep /system/

You will see system method, while with

t.methods.grep /system/

you will see an empty array.

__send__ tries to call the private method inherited from Kernel in the inheritance chain, hence instead of using __send__ use Ruby's public_send method.

Answer 2

If you look up the ancestor chain of your class (Testerize.ancestors), you will find:

[Testerize, Object, Kernel, BasicObject]

Since new classes inherit from Object by default, and Object inherits from Kernel, all of the instance methods available to Kernel are available in your instances.